The impact of downtime on an organization ranges from a minor inconvenience to a total loss of productivity. Some industry averages are reported as high as $80,000 per hour, averaging four hours per event and nine occurrences per year for a loss of nearly $3 million per organization per year. Another survey reported annual losses of $350 thousand to $11 million per organization with an average annual loss of $5 million.
The elimination of downtime would be idea, but not practical. Reducing the length and severity of the downtime is one of the goals of technical training. While doing Motion training at Intellect Controls Group, Inc. we encounter primarily maintenance electricians, as well as engineers and technicians. In teaching these courses we have noticed some common misconceptions or confusion about Motion applications. Sometimes these misunderstandings prolong the troubleshooting process.
One such misunderstanding involves 3 phase motor connections. In conventional 3 phase motor situations, reversing any two of the three motor leads will result in reversal of direction of motor rotation. Interestingly, in motion applications, misconnecting the U, V, and W connections will not result in a reversal of rotation. This will cause the motor to run slow, hot, and perhaps erratically.
What was overlooked was that the motion drives utilize feedback to monitor motor motion. They can compensate to drive the motor in the same direction by electronically reversing the order in which the phases are switched within the drive. Unfortunately they can’t quite do the timing of the switching when this happens and the motor will not run efficiently.
Another overlooked fact is that modern VFD drives will continue to operate with a missing phase operating with two phases instead of three. In fact, they will work with one phase if de-rated. Older drives had no missing phase diagnostic. Usual diagnostics when this occurred included over current faults, under voltage faults, and over temperature faults. These tend to point to a drive problem, not a supply problem.
We ran into this on a robotic application where movement downward would occur, but movement upward would immediately trip out the drive with the previously mentioned faults. The power supply modules were replaced with no results. After some thinking, we realized that this robot was used for training and subject to uncorrected “bugs”.
Sure enough, carefully tracing of the input power circuits revealed a disconnected phase. Of course, in real life, you don’t have “bugged” equipment, but phases do get disconnected for various reasons including tripped breakers or broken connections. Missing phases can even be hard to spot in simple 3 phase motor situations, because the symptoms of running hot, or not developing proper horsepower are not immediately attributed to a missing phase.
Late model drives probably do have a missing phase diagnostic, which may make this even easier to overlook in the older drives because people will assume that all drives have this diagnostic.
Fortunately, detecting a missing phase is simple; just check the voltage between all three phases. They should be the same. Be sure to follow appropriate precautions for the voltages you are measuring.
An area of limited information, and resulting misinformation, regards tuning of a drive. With modern Motion drives this should not need to be done often and with Auto-Tune procedures this should be a simpler procedure than in the past. Most Auto-Tune procedures just involve selecting the parameters to be adjusted and then clicking on a software button to initiate the procedure. You can then tweak the values if you feel it is necessary.
Manufacturers indicate that re-tuning should not be necessary when replacing a motor with a motor with the same part number. Others have their doubts about whether motors are really that identical.
Another argument for re-tuning involves a weak motor or increased bearing resistance as the system ages. Obviously the correct solution to this situation is to replace the motor or bearings. However, at $80,000 per hour, waiting for a replacement motor could be quite costly. Somewhat like PLC’s which can be re-programmed to compensate for mechanical problems and thus allow the postponement of the proper repair until a more convenient time, motion drives can to some degree be tweaked to compensate for mechanical problems.
Tweaking the PLC to compensate for mechanical degradation required “un-tweaking” when the proper repairs were made. Similarly, electronic adjustments to motion systems would require re-tuning after the proper repairs have been made.
When tuning, should we tune with or without the load? With some thought you realize that we are really trying to tune the movement which includes the entire moving mechanism, not just the motor. But with a little more thought you realize that since tuning procedures are going to move the motor and the mechanism, if it’s connected, in a somewhat unknown and vigorous manner, maybe tuning with the load connected isn’t a good idea.
When using Auto Tune, manufacturers suggest a fairly high velocity for movement speed to successfully tune. The problem is that high inertia loads such as flywheels, fans, and pump impellers don’t change speed and position suddenly. In fact sudden changes in speed and position with high inertia loads can result in shaft damage and fan blades bending. So we don’t want to impose sudden changes on these devices.
One approach is to do the initial tuning with the load disconnected to verify motor hookup connections are correct and to achieve initial tuning values. Upon verifying that motor movement is in the direction expected and having achieved initial tuning, then connect the load and with appropriate speed and acceleration limits entered, proceed to tune with the load attached.
Because the situations vary so widely, we can’t give any specific values here. The procedure for your application should be derived by someone knowledgeable with your system. Then it should be written out and documented for others to follow. This procedure should include when to tune, who should be doing the tuning, the specific sequence to follow when tuning, and finally maximum speed and acceleration values to use when tuning with the load connected.
Another area of possible confusion is how to recover from a system failure after it has reached its hard stops. It should never have reached the hard stops, the soft stops should have caught it, so this is considered a very serious fault, and the drive has shut down and disabled itself. You probably don’t want to disable the hard stops and allow it to move farther in the wrong direction, and you can’t clear the fault before you get it off the hard stop. This means you will have to manually move it back into the normal operation range.
Although not recommended, you may be able to move a small system manually without disconnecting the load. However, on larger systems, you may need to disconnect the load in order to reposition it. This needs to be done with the consideration of stored (potential) energy and the resulting movement that is likely to occur with the release of the load. This again is an area where careful thought should be given to how the load will be moved manually, what sources of energy need to be locked out or controlled. Don’t forget that stored energy can be in form of unsupported weight, air or hydraulic pressure, or water pressure. There are many configurations possible so again it is not possible to be system specific, but a well thought out procedure before the emergency can solve a lot of problems. Such a procedure would include, but not be limited to, proper lockout/tagout procedures, proper support of the load, and how the load is to be moved back within its zone. Will we use human power and shove it back into position or will we need mechanical power? If mechanical power, what form? Can we do it with a come-along? Do we need something more powerful? The procedure should also cover the re-connection of the load. How do we align and maintain position while the load is re-connected. A recovery procedure would also need to include steps on homing and then a system check out before returning the equipment to production. You may also want to specify who should do the recovery process. Do you need someone with specific skills or are the procedures and training sufficient enough that all maintenance personnel capable?
These issues are not difficult, and with proper training, the issues of motor connection, missing phase, and when to tune are easy to identify. With good training, procedures and communication, time to recover can be greatly reduced.
Leno Pederson & Howard Loveless
Chief Instructor
INTELLECT CONTROLS GROUP, INC
502-499-7522
Fax 502-499-0862
Instructor/Controls Engineer INTELLECT CONTROLS GROUP, INC
502-499-7522
Fax 502-499-0862
